Fixed example for multiple drives

Here was some example output with multiple partitions:

```
Drive : /dev/sdb1
Total : 61203283968
Used : 49392123904
Free : 8589934592
Drive : /dev/sdb1
Total : 103809024
Used : 35651584
Free : 63963136
Drive : /dev/sdb1
Total : 259845521408
Used : 183609851904
Free : 63350767616
```

The drive name is always the same, so I updated the code.  I think the problem was that the `mydrive` var was getting overwritten before the callback method given to `freedisk.detail` was getting called, thus the `mydrive` appears in the output as the last drive in `drives`.